Most modern electronic equipment rely on circuit boards to route signals among electronic components. A circuit board is typically made of a non-conductive base material. Conductive traces may be printed on the circuit board to couple one electronic component to another. A circuit board may have multiple layers and/or be double-sided so that more components may be packed in a relatively small space. For multi-layered and/or double-sided circuit boards, it is often necessary to create small holes, known as vias, to accommodate electrical connections among different layers or between two sides of the circuit board.
In a circuit board hosting a high data rate circuit, the vias can cause signal integrity problems. FIGS. 1–3 illustrate how a conventional via in a circuit board may be problematic for high data rate transmissions.
FIG. 1 shows a cross-section of a circuit board 100 having a first layer 11, a second layer 12, a third layer 13, and a fourth layer 14. A first trace 102 is formed on the first layer 11 and a second trace 104 is formed on the fourth layer 14. A conventional via interconnect 106 electrically couples the first trace 102 to the second trace 104. The via interconnect 106 may be formed by drilling a through hole 108 from the first layer 11 to the fourth layer 14. The through hole 108 may then be electroplated with a conductive material, typically a metal (e.g., copper). The resulting barrel-shaped via interconnect 106 is also known as a plated through hole (PTH) interconnect.
FIG. 2 is an isolated view of the first trace 102 and the second trace 104 electrically coupled together by the via interconnect 106. The first trace 102, the via interconnect 106, and the second trace 104 form an equivalent path for signal transmissions between point A and point B. If the via interconnect 106 is unfolded, a metal sheet 110, as shown in FIG. 3, may be obtained.
As shown in FIG. 3, the via interconnect 106 (i.e., the metal sheet 110) has a large physical dimension relative to the first trace 102 and the second trace 104. As a result, the via interconnect 106 tends to create excessive capacitance, resulting in an impedance discontinuity in the transmission line between point A and point B. That is, an electrical signal may experience an impedance drop in the via part of the transmission line. This impedance discontinuity may not be a problem for low data rate transmissions. However, at a high data rate, signal integrity may be adversely affected. Further, if a transmission line stitches through a circuit board multiple times passing through several vias, this signal integrity problem may be exacerbated.
In view of the foregoing, it would be desirable to provide a technique for reducing via capacitance which overcomes the above-described inadequacies and shortcomings.